1. Field of the Invention
This invention relates to a projection lens for a projector, and more particularly to a bright projection lens of high definition which is suitable for a projection lens for a television projector of high quality.
2. Description of the Prior Art
A television projector is designed so that a picture plane of a cathode ray tube is enlarged and projected on a screen by a projection lens as is known. A demand of such a projector has been recently increased not only for business use but also for home use and the wide-use thereof has been expected. With this trend, projector devices of more compact size and lower cost have been greatly desired. A projector lens also becomes necessary to have a large aperture ratio, approximately 1:1.2, and an extremely wide angle.
On the other hand, a sign now appears in which a so-called high quality television having a resolution equal to that of a 35 mm movie has been put to practical use mainly for business use, and expected to come into wide use for home use. Under these circumstances, also in the television projector, the demand toward the high quality of image is particularly high, and the image quality of high definition as seen in the high quality television has been desired also for the television projector.
Projector lenses in an attempt of obtaining an image quality of high definition in response to such a demand as described are disclosed as examples thereof in unexamined published Japanese Patent Application Nos. 106427/1987, 85212/1987 and so forth. In these lenses, as can be seen in their examples, an aspherical lens in used for the first lens unit whereby the spherical aberration, the coma and the like due to the aperture are corrected.
Actually, however, the technique for preparing aspherical lenses has not yet been established, and it is very difficult to prepare them even at present. Further, even in the case where an aspherical lens formed from plastic which is relatively simply worked is used, the environmental characteristics are inferior to that of a glass lens. Therefore, particularly in the case where an aspherical lens is used for a portion as the first lens unit in which an error in fabrication greatly influences on aberrations due to the aperture, the spherical aberration is greatly affected, and a higher order flare occurs to lower contrast. As a result, the image quality is deteriorated to make it difficult to realize a projector of high definition.
In view of the foregoing, it is contemplated that a spherical lens is used for the first lens unit in order to solve the aforementioned problem. However, it has been heretofore essential to use aspherical surfaces for the first lens unit in order to realize the image quality of high definition in terms of design. Under these circumstances, a projection lens for a projector capable of correcting various aberrations in a satisfactory manner while using a spherical lens for the first lens unit has been demanded.